Thermal spraying, also known as flame spraying, involves the heat-softening of heat-fusible material, such as a metal or ceramic, and the propelling of the softened material in particulate form against a surface to be coated to which the heat-fusible material bonds. A thermal spray gun is usually used for this purpose and, with one type, the heat-fusible material is supplied in powder form to the gun. The powder is of quite small particle size, e.g., below about 100 mesh U.S. Standard screen size to as small as one micron, and is difficult to meter and control.
A thermal spray gun normally utilizes a combustion or plasma flame to effect melting of the powder, but other heating means, such as electric arcs, resistance heaters or induction heaters can also be used, alone or in combination. In a powder-type combustion thermal spray gun, the carrier gas for the powder can be one of the combustion gases or compressed air. In a plasma spray gun, the carrier gas is generally the same as the primary plasma gas, although other gases such as hydrocarbon are used in special cases.
To obtain high quality coatings, it is necessary to accurately control the rate at which the powder is fed through the gun and to maintain the rate constant for a given set of spray conditions. The type of fine powder used is a very difficult material to handle and to feed with any uniformity into a carrier gas. While various apparatus of different designs and modes of operation based on gravity, mechanical and gas conveying, and combinations thereof, have been proposed such devices almost universally suffer from a lack of reliability in maintaining a constant controlled powder feed rate and are often subject to mechanical wear and breakdown. A contributing factor is the wide range of powder sizes, materials and particle shapes used for thermal spraying.
The present invention pertains to and utilizes a powder feeder broadly of the type described in U.S. Pat. No. 4,561,808. This patent discloses a powder feeding system comprising an enclosed hopper for containing powder in loose particulate form. A carrier gas conduit connected to a carrier gas supply extends through the hopper in its lower portion and continues to a point of powder-carrier gas utilization. The carrier gas conduit has therein powder intake orifices which extend into the hopper below the level of the powder and have a geometric design and arrangement such that there is no gravity flow of the powder therethrough into a carrier gas stream in the carrier gas conduit in the absence of a fluidizing gas flow therethrough.
Fluidizing feed gas at a regulated pressure is supplied to the hopper so that in passing to the orifice the gas must pass through the mass of solids and be diffused thereby. The design of the hopper is such that the gas converges towards the powder intake conduit and fluidizes the powder in a fluidized zone in the immediate vicinity thereof, the powder surrounding the fluidized zone being non-fluidized and acting as a diffusion region for introducing the fluidized gas uniformly into the fluidized zone.
As further disclosed in U.S. Pat. No. 4,561,808, the carrier gas is supplied in a predetermined, constant amount. The flow of the feed gas is regulated by the pressure in the carrier gas conduit, which pressure is responsive to the mass flow rate of solids therethrough. The change in the pressure in the conveying gas line, if any, regulates the flow of the fluidizing gas. Since feed gas pressure is constant, if the carrier conduit pressure should increase, the flow of the fluidizing gas is made to decrease, and vice versa.
It has been found that the type of system of U.S. Pat. No. 4,561,808 has excellent repeatability and uniform control of the powder feed rate and has proven to perform significantly better than predecessor feeders. For example, powder flow rate can be maintained within about 5%. However, for some applications, substantially better precision is required.
Various other devices have been utilized for sensing and controlling powder flow rate by the use of closed loop feedback. For example, U.S. Pat. No. 3,976,332, which discloses a predecessor to the feeder of aforementioned U.S. Pat. No. 4,561,808, teaches the use of a fluidic amplifier that supplies the feed gas to the hopper at a rate controlled by pressure in the carrier conduit. U.S. Pat. No. 2,623,793 discloses a differential pressure controller which detects differential pressure between a powder-gas separator and a gas supply tube at the bottom of a powder-gas lift pipe. The differential pressure controller operates a control valve which supplies gas to a feeder.
U.S. Pat. No. 3,365,242 teaches coarse control of powder flow by adjusting pressure in the gas space above the powder in a tank. Fine control of powder discharge is brought about by a valve which supplies gas to a main powder discharge line. The valve is operated by a pressure response actuator which senses the pressure at the discharge line. As an alternative, instead of pressure monitored discharge rate of powder, a weighing apparatus is mentioned in the patent for delivering a signal responsive to flow of powder.
Other methods are known for measuring powder feed rate. For example, U.S. Pat. No. 4,613,259 discloses the detection of nuclear radiation passed through a tube carrying powder. The detector signal is processed by a computer which controls a mechanical metering device on the feeder. This patent also describes the separate use of a rubber tubing pinch valve for shutting the powder carrier gas flow on and off. The tubing is surrounded by a chamber filled with a fluid at high pressure to pinch off the tube and prevent flow.
German Patent Application 3,211,712 discloses, in tandem, a pair of powder hoppers, a feeding device with an adjustable feed rate, a storage chamber, a variable gap gravity feed from the storage chamber, and a carrier gas injector. Powder falls into the injector through the preset gap. A control device detects the height of the powder in the storage chamber and regulates the feeding device to maintain constant height.
Copending U.S. patent application Ser. No. 927,012 filed Nov. 4, 1986, assigned to the same assignee as the present application, discloses a particular use of a load cell for measuring powder flow rate. An analog signal from the load cell reflects the weight of the hopper containing the powder. The analog signal is conditioned by circuitry which differentiates the signal to a rate of change, filters out transient signals and provides a suitably scaled output for feed rate indication. Time constant is normally of the order of several seconds to one minute in order to prevent clutter by extraneous transients, and the circuitry provides for quick response to major changes in flow rate. This system provides an excellent measure of average flow rate, but the time constant is too long to be utilized alone in a feedback loop where rapid control of powder flow fluctuations is desired.
Therefore an object of the present invention is to provide an improved powder feeding system and method having a high degree of regulation of powder flow rate with a quick response time.
A further object is to provide novel closed loop regulation of powder flow rate in a powder feeding system of the type having a carrier conduit with an intake orifice for entraining powder from a hopper in the presence of feed gas to the hopper.